The IEEE (Institute of Electrical and Electronics Engineers) 802.16 standards propose using Orthogonal Frequency Division Multiple Access (OFDMA) for transmission of data over an air interface. OFDMA has also been proposed for use in 3GPP (Third Generation Partnership Project) Evolution communication systems. In an OFDMA communication system, a frequency bandwidth is split into multiple contiguous frequency sub-bands, each sub-band comprising multiple sub-carriers, that are transmitted simultaneously. A user may then be assigned one or more of the frequency sub-bands for an exchange of user information, thereby permitting multiple users to transmit simultaneously on the different sub-carriers. These sub-carriers are orthogonal to each other, and thus intra-cell interference is minimized.
In order to maximize bandwidth usage, for any given Transmission Time Interval (TTI) the sub-bands may be allocated to users based on measured channel conditions. Further, an appropriate modulation scheme and coding scheme may be determined for each sub-band and each TTI based on the measured channel conditions. The channel condition measurements are performed by a mobile station (MS). The MS measures channel conditions, such as a Signal power over the Interference plus Noise Ratio (SINR) for a common pilot channel or on a preamble, for each and every sub-band during a measuring period, such as a Transmission Time Interval (TTI) (also known as a sub-frame) or a radio frame transmission period, and then reports an average of the measured channel conditions across all of the sub-bands to a serving Node B in a Channel Quality Information (CQI) message. Based on the reported average, or mean, SINR, an OFDMA communication system selectively schedules the sub-bands over a scheduling period, typically one or more TTIs or radio frames, and further adaptively determines appropriate modulation and coding schemes for each sub-band during the scheduling period.
Typically, the CQI message comprises six bits borrowed from a system overhead message. For example, FIG. 1 is a table 100 depicting a mapping of modulation schemes and coding rates to mean SINR reported in a CQI message. The first column 101 of the table lists the mean SINR values that may be reported by a MS (wherein mean SINR of ≦0 dB may be represented by a single value) and the second, third, and fourth columns 102, 103, and 104 of the table 100 list the modulation schemes, error encoding rates, and repetition rates mapped by a scheduler to the corresponding mean SINRs values reported by the MS. However, reporting a mean SINR across all sub-bands may not describe the channel properly, especially with frequency selective fading channels.
Therefore, a need exists for a method and apparatus that provides channel quality information sufficient to construct a fading profile of a frequency bandwidth with fading channels and that does not consuming the overhead resulting from the reporting of CQI for every sub-band of the frequency bandwidth.
One of ordinary skill in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Also, common and well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.